util/nvramtool/compute_ip_checksum.c


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/*****************************************************************************\
 * compute_ip_checksum.c
\*****************************************************************************/

#include <stdint.h>
#include "ip_checksum.h"

/* Note: The contents of this file were borrowed from the coreboot source
 *       code which may be obtained from http://www.coreboot.org.
 *       Specifically, this code was obtained from coreboot (LinuxBIOS)
 *       version 1.0.0.8.
 */

unsigned long compute_ip_checksum(void *addr, unsigned long length)
{
        uint8_t *ptr;
        volatile union {
                uint8_t  byte[2];
                uint16_t word;
        } value;
        unsigned long sum;
        unsigned long i;
        /* In the most straight forward way possible,
         * compute an ip style checksum.
         */
        sum = 0;
        ptr = addr;
        for(i = 0; i < length; i++) {
                unsigned long value;
                value = ptr[i];
                if (i & 1) {
                        value <<= 8;
                }
                /* Add the new value */
                sum += value;
                /* Wrap around the carry */
                if (sum > 0xFFFF) {
                        sum = (sum + (sum >> 16)) & 0xFFFF;
                }
        }
        value.byte[0] = sum & 0xff;
        value.byte[1] = (sum >> 8) & 0xff;
        return (~value.word) & 0xFFFF;
}